|Publication number||US6975393 B2|
|Application number||US 10/386,584|
|Publication date||Dec 13, 2005|
|Filing date||Mar 11, 2003|
|Priority date||Mar 11, 2003|
|Also published as||US20040179187|
|Publication number||10386584, 386584, US 6975393 B2, US 6975393B2, US-B2-6975393, US6975393 B2, US6975393B2|
|Original Assignee||Verity Instruments, Inc.|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (5), Referenced by (2), Classifications (11), Legal Events (10)|
|External Links: USPTO, USPTO Assignment, Espacenet|
The present application claims priority from PCT Application No. PCT/US01/28271 entitled “Afterglow Emission Spectroscopy Monitor” filed on Sep. 10, 2001, currently pending, which claims priority from U.S. Provisional Patent Application No. 60/231,588 filed on Sep. 11, 2000 and U.S. Provisional Patent Application No. 60/266,763 filed on Feb. 5, 2001. The above-identified applications are incorporated by reference herein in their entirety.
1. Field of the Invention
The present invention relates to semiconductor manufacturing, monitoring of vacuum chamber cleanup and trace gas analysis.
2. Description of Related Art
The semiconductor manufacturing industry achieves higher and higher levels of integration of integrated circuits dealing with contamination and yield problems. Frequent pump/purge cycles with high purity purge gas in between reactive process gas cycles are hereto applied. These pump/purge cycles clean out process gas from a prior reactive process cycle, as well as species like oxygen and moisture that might have entered the tool's chamber. In-situ, real time process monitors of all kind of process variables are considered critical for current and future developments. An example of a new type of monitor is the measurement of trace concentrations of moisture in the exhaust of a semiconductor-manufacturing tool by diode laser based absorption spectroscopy.
Traditionally, monitoring the pressure during pumpdown can reveal gross problems with leaks or excessive outgassing. A pressure gauge, however, is not specific for a particular species. Moreover, most information is in the very tail part of a drydown curve and, in a normal operating procedure, there is simply no time to wait that long. The purge and pump cycles are often done simultaneously, opening wide to the pump while entering a stream of purge gas with a total cycle time in the order of 10 or 20 seconds.
Species specific monitoring can be done with a mass spectrometer that is then called a residual gas analyzer (RGA). Such RGA operates at 10−5–10−6 Torr requiring differential pumping when probing a purge gas at a higher pressure. Mass spectrometers have a residual spectrum at the lower masses that prevents measuring water and oxygen at levels below a part per million.
Some of the above and other in-situ measurement techniques for pump/purge cycles can detect the removal of previously introduced reactive process gas and rare intrusions of excessively high contamination levels. Even to accomplish this limited objective, in-situ techniques, such as RGA, are expensive, maintenance intensive and require elaborate calibration. Moreover, none are sensitive enough and fast enough to follow the actual progress of mentioned 10–20 seconds pump/purge cycles at partial pressures of relevant contaminants in the order of 10–12 atmosphere.
Optical (none afterglow) emission spectroscopy finds practical applications in the semiconductor manufacturing industry. The observation of the spectrum emitted by the process plasma during a process and its evolution over time provides information about the completion of certain reactions such as in end point detection. The presence of a broad background spectrum, however, obscures any weak emission from trace species.
Non-in-situ qualification of cylinder gas or bulk supply of electronic grade nitrogen and argon by metastable transfer emission spectroscopy (MTES) is investigated for oxygen-containing impurities by J. W. Mitchell et al., Analytical Chemistry 1986, 58, 371–374, which is incorporated by reference herein in its entirety. The required bulky and maintenance intensive vacuum technology in such a flowing afterglow application of MTES prevents it from being applied in any practical gas analytical instrumentation.
Non-in-situ gas analysis at parts per billion levels is routinely performed on line for the purge gas prior to its entering into the process tool. At that point, the purge gas is at higher than atmospheric pressure while the analysis can be performed in the order of minutes. The very sensitive and contaminant specific instruments for such purge gas analysis are used downstream of the large purifiers that provide gas to a semiconductor plant's gas distribution manifold. Such instruments are still too complex and expensive to be used as simple end of life detection for point of use purifiers that are used at the purge gas inlet of an individual tool.
Such non-in-situ analysis prior to entering the tool can sometimes also be performed for the reactive process gases although, highly desirable hygrometry of corrosive gases can only be done economically per cylinder, off line, in a laboratory environment.
Even non-in-situ applications requiring only ppm level detection limits can be troubled by the lack of speed, reliability, robustness and ease of maintenance of current state-of-the-art instrumentation. Such applications include a 10-percent increased efficiency of the recovery of argon from an air separation system described in U.S. Pat. No. 4,784,677 entitled “Process and Apparatus for Controlling Argon Column Feedstreams” by Al-Chalabi, and U.S. Pat. No. 5,448,893 entitled “Process for Maximizing the Recovery of Argon From an Air Separation System at High Argon Recovery Rates” by Howard et al., both of which are incorporated by reference herein in their entireties, requiring ideally an analyzer with a one-second response time. U.S. Pat. No. 4,801,209 entitled “Process and Apparatus for Analyzing a Gaseous Mixture and a Visible Emission Spectrum Generator Therefor” by Wadlow, which is incorporated by reference herein in its entirety, describes an analyzer for the maximization of argon recovery, but lacks the robustness and reliability needed for application in a process environment.
Accordingly, besides the objects and advantages of the monitors described above, several objects and advantages of the present invention are to provide a device and method for sensitive and rapid in-situ gas contaminant monitoring during the pump/purge cycle of vacuum tool equipment where such monitoring includes, among others:
Other objects and advantages are to provide devices and methods to perform non-in-situ gas analysis in a simple, robust and economical fashion for applications such as end of life detection for gas purifiers and in air distillation applications requiring respectively very low detection limits and very fast response times.
Afterglow spectroscopy allows observation of light emission of gaseous species in the absence of direct plasma light. This absence avoids the creation of a background spectrum obscuring weak emission from trace species. The invention describes a flowing afterglow version monitoring in-situ the cleanup of vacuum tools during pump/purge cycles. The invention also describes an intermittent afterglow version suitable for trace gas analysis at atmospheric pressure.
The novel features believed characteristic of the present invention are set forth in the appended claims. However, the invention itself as well as a preferred mode of use, further objectives and advantages thereof, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings wherein:
Other features of the present invention will be apparent from the accompanying drawings and from the following detailed description.
A first exemplary embodiment of the present invention, depicting afterglow monitor on tool exhaust, is shown in
More arms can be added to tee piece 9 to observe afterglow emission at additional wavelengths through additional windows and interference filters. The geometry for mounting such additional arms should avoid direct plasma 3 light from reaching the arm's observation window while a lens is used to focus light from the afterglow on the corresponding multiplier.
The inside of aluminum vacuum pieces such as tee 9 can be black anodized to absorb direct plasma light, thus reducing the light reaching the observation window through reflections.
Quartz tube 4 in between plasma 3 and electrodes 5 and 6 avoids potential contamination from contact of the metal electrodes with the plasma.
Optionally, the first exemplary embodiment of the present invention can, instead of using its own plasma 3 source, use the plasma of the process tool itself. This is achieved by igniting (after an impedance match) the tool's plasma in a purge gas stream of inert gas like nitrogen or argon.
A second exemplary embodiment of the present invention, depicting afterglow monitor with DC plasma, is shown in
A third exemplary embodiment of the present invention, depicting afterglow monitor at the tool's purge gas inlet, is shown in
Besides impurities already present in the purge gas itself, the third exemplary embodiment of the present invention allows observation of species from inside the tool's process vacuum chamber that back diffuse into the stream of metastables that exits the monitor. Such backdiffused species will react with these metastables and resulting light emission can be monitored by the afterglow monitor. These backdiffusing species are the indicators of the quality of the ongoing purge.
Apart from purge gas, also part of the process gas constituents, like argon, can be made to enter the tool passing through the afterglow monitor during reactive process cycles. This will protect the monitor's observation window 10 from process tool plasma deposits that will now have to diffuse against a “counterflow.”
Optionally not shown in the
A fourth exemplary embodiment of the present invention, depicting afterglow monitor with zero reference, is shown in
The afterglow monitor in the first exemplary embodiment and the second of the present invention uses the mechanical fore pump from the tool itself. This fourth exemplary embodiment has an afterglow monitor with its own mechanical fore pump 51. A stream of purge gas exiting tool 50 is teed off from the gas stream between the tool's turbo pump 52 exit 53 and the tool's mechanical fore pump 54. Isolation valve 55 can block this teed off stream which is then replaced by a gas stream of purge gas entered directly through a Mass Flow Controller (MFC) 56 in between the afterglow monitor's inlet and isolation valve 55. Purge gas to purge the tool enters through MFC 57, while process gas enters through MFC 58. When the isolation valve 55 is closed, e.g. during active non-purge process cycles, mentioned zero reference can be obtained while a feedback loop tunes MFC 56 to reproduce the same pressure gauge 59 reading that is present during regular purge gas monitoring conditions with isolation valve 55 opened.
In fact, with an opened isolation valve 55 during a tool 50 purge, MFC 56 can be tuned over a range of flowrates while MFC 57 is adapted to maintain a fixed pressure at the indicated pressure gauge 59. This range for MFC 56's flowrate can vary from, on one extreme, a minimum to prevent a stagnant flow at the MFC 56's outlet connection. This corresponds to monitoring undiluted purge gas exiting tool 50. On the other extreme, MFC 56's flowrate can be made large enough to provide not only a flow through the afterglow monitor into the afterglow monitor's mechanical forepump 51, but also an excess flow streaming away from the afterglow monitor mixing in with the purge gas exiting tool 50 toward mechanical forepump 54 of tool 50. This situation would correspond to monitoring undiluted purge gas without contributions from tool 50 that would have to diffuse against the excess purge gas flow. In between these extremes, mixtures of pure purge gas and purge gas with tool contributions can be generated where the ratios can be determined from the MFC 56 flowrate and a measurement of the flowrate at the exit of the afterglow monitor's mechanical forepump 51. Such setup enables to dilute very contaminated purge gas exiting the tool 50 thus assuring measurement on the linear part of the calibration curves. Optionally, this last described setup could function also without isolation valve 55 or with a venetian blind shutter instead of isolation valve 55 to avoid potential contamination during active process cycles.
The isolation valve 55 setup, however, enables a single afterglow monitor with a single corresponding MFC 56 to be connected to several tools of, e.g. a cluster tool, serving one at the time, where each cluster tool has its own isolation valve 55.
Fifth exemplary embodiment of the present invention: intermittent afterglow monitor.
A low frequency square wave generator 71's signal 72 is offered to the lock-in 73 as reference signal, while it is also offered to actuator 74 of mechanical shutter 70 and to the amplitude modulation input of a RF signal generator 75. In case of, e.g. a mechanical rotating chopper wheel, such low frequency square wave signal can be taken directly from the chopper and offered to the lock-in's reference input and the RF generator 75's AM input.
RF generator 75 will produce periods of RF signal 76 in phase with the periods of a closed shutter (chopper). RF generator 75's output will be amplified by amplifier 77 and offered to electrodes 5 and 6 of the plasma. Periods of open chopper 70 will result in afterglow signals from the photo multipliers 42.and 44. A closed chopper photo multiplier signal will correspond, as discussed, with the dark count of the multiplier. Lock-in 73 will produce an output signal proportional to the average signal contribution from the afterglow emission during a chopper 70 period.
Alternative methods of data treatment consist of curve fitting a down-going exponential to the signal obtained as a function of time during the individual open chopper periods. Other options would be to ignore the closed shutter signals and produce a signal proportional to the average emission received during the open shutter periods using gated electronics. If the shutter adequately isolates plasma light, one could also output the averaged received light from the photo multiplier(s).
If necessary, any of the described afterglow monitor exemplary embodiments can be isolated from vibrating components such as the mechanical forepump by, e.g., bellows in order to protect more delicate parts such as a photomultiplier.
Background spectrum: Optical plasma emission spectroscopy is typically not suited to measuring trace concentrations of species. Even high-resolution spectroscopy is of limited use in case of trace concentrations because of the presence of a broad background spectrum. The invention is based on the avoidance of this background spectrum made possible by the realization that it is caused mainly by fluorescence of impurities in the (quartz) observation window/lens 10 irradiated by strong main (UV) lines of the plasma. Apart from this fluorescence, the presence of strong plasma lines and possibly other plasma emissions make it also difficult to detect weak signals from trace concentrations in the plasma.
Shielding of the window: The invention shields observation window/lens 10 from direct plasma emission by creating a “dark” area in front of it. This is done, on the one hand, by geometrical shielding with one or more screens blocking a straight path from the plasma to window 10. Additional blackening of the screens and parts of the chamber can be used to reduce plasma (UV) light reaching window 10 after one or more reflections. Additionally, dark zones created in the plasma, e.g., near the electrodes in a DC plasma or with grounded plates, can also be used to limit direct emission from irradiating the window. These forms of shielding are applied for a plasma that is continuously ignited during, for example, purge cycles of a semiconductor manufacturing process, in contrast to the hereafter discussed intermittent plasma.
On the other hand, observation window or lens 10 can be shielded from plasma light by means of mechanical shutter 70 positioned in between window 10 and the plasma. Extinguishing the plasma and opening shutter 70 allows the observation of the afterglow emission decreasing over time. Shutter 70 can be closed again and the plasma reignited whereupon the cycle is repeated. Such intermittent cycles can be repeated many times, e.g., during one semiconductor manufacturing purge cycle. This form of shielding is applied in the invention's intermittent plasma afterglow monitoring.
Afterglow: Long lived high energetic species, such as metastables, created previously in the plasma end up in front of observation window 10 either by travel in case of the flowing afterglow or by the removal of the chopper barrier in case of the intermittent afterglow. In the flowing afterglow mentioned, travel into the invention's shielded “dark” area can take place either by moving with a stream of gas by diffusing into a stagnant flow or even by moving against a stream of gas (back diffusion). The mean free path without collision of a species in a low-pressure environment can become relatively long, while an excited argon atom cannot lose its energy efficiently colliding with another argon.
The afterglow spectrum observed in isolation of the main plasma has a much less complex emission spectrum than the main plasma itself. The dominant mechanism of excitation of an atom or molecular species by high energetic electrons is replaced by recombination and energy transfer by other atoms, ions or molecules. This results in a much less dense matrix gas spectrum where trace impurity emission appears more clearly in absence of mentioned broad background spectrum.
Interpretation of the spectrum: It is typically difficult to extract quantitative information regarding the type and concentrations of contaminating species present prior to their entrance into the plasma from the information regarding the resulting species observed in the plasma. The highly energetic plasma environment decomposes molecular species that recombine into new species. As an illustrative example, a plasma of argon with 2 ppm H2 and 1 ppm O2 will have similar OH emission as a plasma with 2 ppm H2O.
Another difficulty is that quantitative emission spectroscopy not only involves the concentration of emitting species, but also the intensity of the source of excitation. The invention realizes that a plasma created in ultra high purity purge gases, such as argon or nitrogen used in semiconductor manufacturing tools, addresses both difficulties. On the one hand, such gases decompose in a very limited number of fragments while there is only a handful of impurity types. On the other hand, such plasma provides a stable source of excitation little impacted by the presence of the very low trace concentrations of impurities. As a result, calibration curves will be linear at low concentrations.
The invention can use chemometrics to address remaining non-specificity, basically solving n unknown trace concentrations from n, or more, quantified emission line intensities. The presently-described invention can use additional distinguishing characteristics that are presented hereunder.
Additional distinguishing characteristics: Moisture and oxygen are the main contaminants in most semiconductor manufacturing processes and typically have a negative impact on the process yield. Removal of these two impurities is also the main goal of the pump/purge cycles performed in between active process cycles. In contrast to a molecule like oxygen, a moisture molecule is very “sticky” and has a tendency to reside on the walls of a process chamber. This characteristic makes that during an evacuation, or purge, oxygen is removed relatively fast while moisture is removed much slower.
The way impurities such as moisture and oxygen are introduced into the process chamber also results in different behavior. Oxygen is typically introduced as a leak from the atmosphere into the chamber. Moisture typically outgases from chamber components like O-rings or from materials from, e.g., wafer trays, brought into the chamber. This makes a difference in pumpdown speed, there is also a difference in asymptotic value approached during prolonged purging. An oxygen leak will create a non-zero plateau, while moisture will eventually reach zero.
Presented additional distinguishing characteristics allow mathematical treatment of data obtained as a function of time. Such mathematical treatment can be, but is not limited to, checking if higher derivatives approach zero or, e.g., curve fitting with one or more downgoing exponentials, etc.
Indirect detection of wet process gas: Hygrometry in corrosive gasses is typically problematic because it exposes delicate sensor parts to corrosion. The present invention allows for monitoring the wetness of the process gas used in the cycle prior to a purge cycle. Moisture, as a very sticky molecule, entered into the process chamber by, e.g., wet HCl, will stick long enough to the walls of the chamber to be noticeable by afterglow monitoring in the following purge cycle. Means to distinguish such moisture from that of other sources can be: enter process gas without ignition of the process plasma, vary the introduction time or flowrate of the process gas, and differences with and without a stack of wafers present in the tool chamber.
Pump/purge efficiency monitoring: As mentioned, the purpose of the pump/purge cycles in a semiconductor manufacturing process is typically to remove such trace impurities as moisture and oxygen. In absence of real time data regarding the efficiency of an ongoing pump/purge cycle, a tool operator has to rely on prior experimentally determined purge times that proved adequate under most “normal” operating conditions. These purge times will be applied plus an additional safety margin. In situ metrology will not only cut excess purge time that will become available as process time. It will also catch contamination problems as soon as they occur, thus avoiding time and material costs related to the manufacture of unusable product runs. In-situ monitoring can also cut startup time after maintenance and save costs running test wafers.
Even when used only as a qualitative monitor, the invention allows following the evaluation of characteristic emission line intensities during the pump/purge cycle, which provides a measure for the reproducibility of this part of the process. It can also give an indication whether or not further purging is worthwhile.
Calibration: Calibration of the monitor can be done by the introduction of a purge gas with known concentrations of, e.g., oxygen and moisture. A stable flowrate of such gas can be created at various pressures in the range of interest.
Using the linearity of the monitor's response at low concentrations and the absence of a background spectrum, one can also add calibrated concentrations to an unknown purge gas. Extrapolation allows determining the unknown purge gas concentration.
Using the large volume of the tool chamber upstream of the afterglow monitor, exponential dilution can be used to generate a large range of calibrated concentrations as a function of time. Hereto, a known concentration of, e.g., oxygen or moisture, in high purity purge nitrogen, is flowing until stable afterglow emission is reached in an equilibrium situation. Then a switch introducing only a flow of high purity purge gas is made into the tool whereupon the monitored concentration at the outlet of the tool will follow an exponentially down going function of time. This function can be calculated from the volume of the chamber, the initial concentration, and the size of the flowrate of the purge gas.
Window depositions: The transparency of an observation window in a vacuum chamber used to perform active process steps like chemical vapor deposition might degrade over time. Solutions preventing window deposition range from the creation of a counter gas flow away from the window to window shutters and “venetian blinds”, to an isolation valve to protect the window during active process cycles.
Compensation for window depositions can be done in the following further described setups that allow creating a zero or reference emission spectrum. This zero or reference spectrum can be used to “normalize” other measurements. Results can be based on the ratio of line intensities and the zero spectrum line intensities, thus compensating for changes in the transparency of the observation window.
Normalization: As previously mentioned, quantitative emission spectroscopy involve not only the concentration of the emitting species, but also the intensity of the source of the excitation. In case there are emission lines available that are a measurement of the intensity of the source of excitation, such lines can be used to “normalize” a quantitative measurement. This allows for correction in fluctuations in the intensity of the source of excitation. An example according to the invention is the use of afterglow emission caused by transitions from N2(C3Πu) to N2(B3Πg) in a nitrogen plasma. Two N2(A3Σ) metastable nitrogen molecules can react with each other leaving one of the molecules in the (C3Πu) state. This makes the concentration of metastable N2(A3Σ) proportional to the square root of the intensity of mentioned afterglow emission (N2(C3Πu) to N2(B3Πg)).
Metastable production: Plasma conditions such as DC voltage and current or AC frequency, voltage and current can be tuned to generate high concentrations of metastable species in the plasma. Metastables can loose their energy either by wall collisions or by collisions with trace impurities. The rate of losses of metastables through wall collisions depends on the diameter of the tubing, the flowrate of the gas, pressure, diffusion coefficient, concentration of metastables generated in the plasma and the distance downstream of the plasma. Energy loss through collision with trace impurities depends on the concentration of the impurities and the reaction rate. The metastable concentration will be stable at low impurity concentrations resulting in a linear calibration curve. Such curve will eventually flatten at high impurity concentrations that deplete the metastable concentration making less metastables available to generate afterglow emission.
Emission spectra: The afterglow emission is typically generated by highly excited diatomic species which can be molecules or radicals created mostly from recombining fragments of species decomposed in the plasma. The emission spectrum associated with a particular transition between specific molecular states appears in the form of bands containing peaks each associated with a specific vibrational initial and final state. For the purposes of this invention, a particular wavelength has to be selected, e.g., when an interference filter is used. In principle, any major peak, or group of peaks, from a band can be chosen for the monitoring as long as it is not in the vicinity of possible other afterglow emission wavelengths from another species.
Argon purge gas: The metastable state of Argon3P2 is typically the main source of excitation of the impurity molecules and in a much lesser extent metastable Argon3P0. Emission that can be seen in an Argon afterglow originate from species like, N2(C3Πu)−(B3Πg) and radicals like OH(A2Σ)−(X2Π) and CH(A2Δ)−(X2Π).
As discussed above, the main concern when purging a vacuum tool is the removal of moisture and entered atmospheric components like oxygen. Using ultra high purity Argon as purge gas the afterglow's OH, N2, and eventually NO emission enable to monitor for moisture and air leaks. Leaks from atmospheric air will bring in a fixed well-known ratio of nitrogen and oxygen, making de nitrogen emission a good tracer for the presence of the corresponding oxygen. The NO emission can provide additional information.
Nitrogen purge gas: The N2(A3Π) metastable is the main source of excitation of NO formed in the nitrogen plasma out of oxygen containing species such as moisture and O2. NO emission is caused by transitions from NO(A2Σ+) to NO(X2Π). Other emission lines in a nitrogen purge gas can be OH(A2Σ)−(X2Π) and CN(B2Σ)−(X2Σ). See also the Normalization section.
Bypass the tool: A valve arrangement could send a purge gas stream directly into the afterglow detector bypassing the tool. This allows for comparing the difference in emission from the afterglow with and without the contribution from the tool. Such arrangement can be combined with the use of an isolation valve to protect the afterglow monitor during the actual process cycle. Emission spectra from purge gas bypassing the tool, hereinafter called a zero or reference spectrum, can be obtained during the time periods that an active (non purge) cycle takes place in the tool. In some arrangement (see exemplary embodiment of the present invention), the afterglow plasma can stay on continuously.
Performance: Linear calibration curves were obtained up to the parts per million range, with a darkcount rate of the photomultiplier corresponding to one part per billion (moisture in argon). Interference filters instead of the monochromator used to obtain the above-mentioned sensitivity allow catching a much larger fraction of the emitted light, not only in terms of space angle but also in terms of a wider frequency band. This and an optimized design should easily enable 0.1 ppb detection limits with response times in the order of seconds. In a 2 mBar purge gas, such detection limit represents a partial pressure of 2×10−13 Bar.
Afterglow monitoring of the removal of reactive species introduced prior to the purge/pump cycle: The first exemplary option to do this is simply monitoring afterglow emission associated with remaining reactive species as a function of time during the purge cycle.
A second exemplary option is to use, in addition to the emission of the first option, also emission related to the concentration of purge gas metastable species such as caused by transitions from N2(C3Πu) to N2(B3Πg) in a nitrogen plasma. Similarly, as described above (see the Normalization section), this last emission can now also be used to normalize the reactive species related emission.
The third exemplary option is to monitor just the mentioned emission related to the concentration of purge gas metastable species such as caused by transitions from N2(C3Πu) to N2(B3Πg) in a nitrogen plasma. Apart from losses through wall collisions, the metastable species will lose their energy through quenching. Quenching is caused by metastables colliding with other contamination molecules and with molecules from the remainder of reactive species introduced prior to the purge/pump cycle. The better the reactive species are cleaned out by the purge, the stronger the emission from, e.g., a N2(C3Πu) to N2(B3Πg) transition. This third exemplary option can result in a very simple device where metastables diffuse out of a shielded plasma area into the observation of a photo sensor with an interference filter.
The fourth exemplary option monitors the same emission as the previous third option, but uses the additional information about the strength of this emission at “zero” gas conditions as a reference. Zero gas conditions can be obtained either from the end state of a previous purge/pump cycle or by the above-described purge gas bypassing the tool.
A fifth exemplary option is to operate the flowing afterglow setup in a pulsed fashion. Such an option would require detection optics that gather emissions over a certain length of the path of the afterglow. Such optics can, for example, consist of a photosensor in the focal point of a lens with the observation direction along the flow path of the afterglow. The “flowing” of the afterglow can, as mentioned above, in its simplest form be caused by diffusion of metastables away from a shielded active plasma zone. Monitoring the dying out in time of any afterglow emission following a plasma pulse will reveal information about the presence of quenching species such as the remainder of previously introduced reactive process gas.
A sixth exemplary option combines the fifth exemplary option with information of dying out characteristics of zero gas species following a plasma pulse. This zero gas information can again be used as a reference corresponding to an ideally cleaned tool.
Intermittent afterglow arrangement: An alternative to the hitherto discussed continuous afterglow arrangements is the intermittent afterglow arrangement. In the intermittent afterglow arrangement, the plasma is alternately ignited and extinguished. During plasma ignition, emission is blocked from reaching the observation window by a chopper provision placed in between the window and the plasma. When the plasma is extinguished, the chopper provision clears access to the window. In this way, afterglow emission can be observed in absence of direct plasma light and in absence of fluorescence of impurities in the observation window.
The plasma is created in a volume in front of the chopper provision and might be sustained during most of the blocking period of the chopper. Losses of metastables by wall collision are limited, while phase sensitive lock-in detection can be applied allowing the measurement of afterglow emissions smaller than the darkcurrent of the photomultiplier.
For in-situ monitoring in a vacuum tool however, the mechanical shutter operates under relatively difficult conditions being placed in a vacuum and exposed to a plasma.
However, the intermittent afterglow using, e.g., a fast enough rotating perforated disk chopper can observe the very short-lived afterglow associated with operation at higher gas pressures such as atmospheric pressure. Such a device can perform non-in-situ gas analysis on gases like argon, nitrogen and hydrogen at pressures around atmospheric pressure making it unnecessary to use any vacuum equipment such as a vacuum tight window, pumps etc. The plasma in this case should be very short lived and can be created just by an arc discharge much like the ignition in a spark plug. The position of the rotating disk chopper will trigger the discharge similar to the way as a car's ignition system is triggered by the proper position of the pistons. The chopper disc and surrounding chamber can be made of metal to act as a Faraday cage blocking potentially perturbing electromagnetic radiation when igniting the discharge.
Application involving long lived excited state species: So far, the presently disclosed afterglow emission resulted mainly from energy transfer from non-emitting metastable species to contamination molecules. The presently-described invention can also utilize emissions from long-lived species created in the plasma. An application according to an exemplary embodiment of the present invention is the detection of traces of nitrogen in a matrix of pure oxygen or a matrix of a mixture of oxygen and argon. Nitrogen and oxygen present in a plasma discharge will lead to the formation of species like excited state nitrogen dioxide. Monitoring characteristic nitrogen dioxide emission in the afterglow will be a measurement for the presence of trace nitrogen.
Similarly, traces of oxygen in a nitrogen matrix can be detected. The above-described high pressure intermittent afterglow, according to the invention, is well suited for applications that can be found in air distillation plants described in U.S. Pat. Nos. 4,784,677 and 5,448,893.
Thus, the reader will see that the afterglow emission spectroscopy in situ monitor can rapidly determine the status of an ongoing pump/purge cycle in vacuum equipment. It can determine the level of cleanup of previously entered reactive gas, and indicate the wetness of such gas when it was entered. It cannot report only unusual high levels of contaminants in the purge gas, but it is also sensitive enough to monitor the presence of contaminants such as oxygen and moisture at trace levels. This last capability provides the data to a tool operator that the pump/purge cycle is actually removing contaminants at levels that matter, or is taking painfully long to lower the last few insignificant ppb. Distinguishing the last could make purge time safety margins available as process time for the tool, thereby increasing the overall efficiency. The afterglow emission spectroscopy in situ monitor is simple and compatible with the materials used in a high vacuum and plasma environment, while at the same time it is easy to calibrate and maintain.
While the above description contains many specificities, these should not be construed as limitations on the scope of the invention, but rather as an exemplification of one exemplary embodiment of the present invention thereof. Many other variations in operating conditions and applications are possible. For example, an alternative to a spark generated plasma in the atmospheric pressure intermittent afterglow can be a plasma generated by a short high energetic laser pulse concentrated in the focal point of a lens. When the laser beam and the afterglow observation axis are perpendicular to each other, there will be no observation of eventual fluorescence induced in the laser lens.
Accordingly, the scope of the invention should be determined not only by the exemplary embodiment(s) illustrated, but also by the appended claims and their legal equivalents. The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present invention has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the invention in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the invention. The embodiment was chosen and described in order to best explain the principles of the invention and the practical application, and to enable others of ordinary skill in the art to understand the invention for various embodiments with various modifications as are suited to the particular use contemplated.
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|U.S. Classification||356/311, 356/72, 356/316, 118/715|
|Cooperative Classification||G01N21/68, H01J37/32935, H01J37/32981|
|European Classification||H01J37/32S4, H01J37/32S4J, G01N21/68|
|Mar 11, 2003||AS||Assignment|
Owner name: VERITY INSTRUMENTS, INC., TEXAS
Free format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNOR:METTES, JACOB;REEL/FRAME:013871/0188
Effective date: 20030311
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|Nov 11, 2009||SULP||Surcharge for late payment|
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|Feb 4, 2014||FP||Expired due to failure to pay maintenance fee|
Effective date: 20131213
|Oct 5, 2015||PRDP||Patent reinstated due to the acceptance of a late maintenance fee|
Effective date: 20151005
|Oct 5, 2015||FPAY||Fee payment|
Year of fee payment: 8